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Uric acid
| ChemSpiderID = 1142 | RTECS = }} | Section2 = }} Uric acid (or urate) is an organic compound of carbon, nitrogen, oxygen and hydrogen with the formula C5H4N4O3. Biology Uric acid is produced by xanthine oxidase from xanthine and hypoxanthine, which in turn are produced from purine. Uric acid is more toxic to tissues than either xanthine or hypoxanthine. In humans and higher primates, uric acid is the final oxidation (breakdown) product of purine metabolism and is excreted in urine. In most other mammals, the enzyme uricase further oxidizes uric acid to allantoin.Purine and Pyrimidine Metabolism The loss of uricase in higher primates parallels the similar loss of the ability to synthesize ascorbic acid. Both uric acid and ascorbic acid are strong reducing agents (electron donors) and potent antioxidants. In humans, over half the antioxidant capacity of blood plasma comes from uric acid. The Dalmatian dog has a genetic defect in uric acid uptake by the liver, resulting in decreased conversion to allantoin, so this breed excretes uric acid, and not allantoin, in the urine. In birds and reptiles, and in some desert dwelling mammals (eg kangaroo rat), uric acid also is the end product of purine metabolism, but it is excreted in feces as a dry mass. This involves a complex metabolic pathway that is energetically costly in comparison to processing of other nitrogenous wastes such as urea or ammonia, but has the advantage of reducing water loss. In humans, about 70% of daily uric acid disposal occurs via the kidneys, and in 5-25% of humans impaired renal (kidney) excretion leads to hyperuricemia. Genetics A proportion of people have mutations in the proteins responsible for the excretion of uric acid by the kidneys. Four genes have so far been identified: SLC22A12, SLC2A9, ABCG2 and SLC17A3). SLC2A9 is known to transport both uric acid and fructose. Medicine In human blood plasma, the reference range of uric acid is between 3.6 mg/dL (~214µmol/L) and 8.3 mg/dL (~494µmol/L) (1mg/dL=59.48 µmol/L).SI Units for Clinical Data This range is considered normal by the American Medical Association. Uric acid concentrations in blood plasma above and below the normal range are known, respectively, as hyperuricemia and hypouricemia. Similarly, uric acid concentrations in urine above and below normal are known as hyperuricosuria and hypouricosuria. Such abnormal concentrations of uric acid are not medical conditions, but are associated with a variety of medical conditions. , comparing blood content of uric acid (shown in green) with other constituents.]] High uric acid Gout Excess serum accumulation of uric acid can lead to a type of arthritis known as gout. Elevated serum uric acid (hyperuricemia) can result from high intake of purine-rich foods, high fructose intake (regardless of fructose's low glycemic index (GI) value) and/or impaired excretion by the kidneys. Saturation levels of uric acid in blood may result in one form of kidney stones when the urate crystallizes in the kidney. These uric acid stones are radiolucent and so do not appear on an abdominal plain x-ray or CT scan. Their presence must be diagnosed by ultrasound for this reason. Very large stones may be detected on x-ray by their displacement of the surrounding kidney tissues. Some patients with gout eventually get uric kidney stones. Gout can occur where serum uric acid levels are as low as 6 mg/dL (~357µmol/L), but an individual can have serum values as high as 9.5 mg/dL (~565µmol/L) and not have gout (no abstract available; levels reported at Uric Acid, Serum). Lesch-Nyhan syndrome Lesch-Nyhan syndrome, an extremely rare inherited disorder, is also associated with very high serum uric acid levels. Spasticity, involuntary movement and cognitive retardation as well as manifestations of gout are seen in cases of this syndrome. Cardiovascular disease Although uric acid can act as an antioxidant, excess serum accumulation is often associated with cardiovascular disease. It is not known whether this is causative (e.g., by acting as a prooxidant ) or a protective reaction taking advantage of urate's antioxidant properties. Diabetes The association of high serum uric acid with insulin resistance has been known since the early part of the 20th century, nevertheless, recognition of high serum uric acid as a risk factor for diabetes has been a matter of debate. In fact, hyperuricemia has always been presumed to be a consequence of insulin resistance rather than its precursor . However, it was shown in a prospective follow-up study that high serum uric acid is associated with higher risk of type 2 diabetes independent of obesity, dyslipidemia, and hypertension . Metabolic syndrome Hyperuricemia is associated with components of metabolic syndrome and it has been debated for a while to be a component of it. It has been shown in a recent study that fructose-induced hyperuricemia may play a pathogenic role in the metabolic syndrome. This agrees with the increased consumption of fructose-base drinks in recent decades and the epidemic of diabetes and obesity . Uric acid stone formation Uric acid stones, which form in the absence of secondary causes such as chronic diarrhea, vigorous exercise, dehydration, and animal protein loading, are felt to be secondary to obesity and insulin resistance seen in metabolic syndrome. Increased dietary acid leads to increased endogenous acid production in the liver and muscles which in turn leads to an increased acid load to the kidneys. This load is handled more poorly because of renal fat infiltration and insulin resistance which are felt to impair ammonia excretion (a buffer). The urine is therefore quite acidic and uric acid becomes insoluble, crystallizes and stones form. In addition, naturally present promotor and inhibitor factors may be affected. This explains the high prevalence of uric stones and unusually acid urine seen in patients with type 2 diabetes. Uric acid crystals can also promote the formation of calcium oxalate stones, acting as "seed crystals" (heterogenous nucleation). Low uric acid Multiple sclerosis Lower serum values of uric acid have been associated with Multiple Sclerosis. Multiple sclerosis (MS) patients have been found to have serum levels ~194µmol/L, with patients in relapse averaging ~160µmol/L and patients in remission averaging ~230µmol/L. Serum uric acid in healthy controls was ~290µmol/L. Conversion factor: 1mg/dL=59.48 µmol/L A 1998 study completed a statistical analysis of 20 million patient records, comparing serum uric acid values in patients with gout and patients with multiple sclerosis. Almost no overlap between the groups was found. Uric acid has been successfully used in the treatment and prevention of the animal (murine) model of MS. A 2006 study found that elevation of serum uric acid values in multiple sclerosis patients, by oral supplementation with inosine, resulted in lower relapse rates, and no adverse effects. Oxidative stress Uric acid may be a marker of oxidative stress, and may have a potential therapeutic role as an antioxidant. On the other hand, like other strong reducing substances such as ascorbate, uric acid can also act as a prooxidant, particularly at elevated levels. Thus, it is unclear whether elevated levels of uric acid in diseases associated with oxidative stress such as stroke and atherosclerosis are a protective response or a primary cause.Free Radicals and Human Disease For example, some researchers propose that hyperuricemia-induced oxidative stress is a cause of metabolic syndrome. On the other hand, plasma uric acid levels correlate with longevity in primates and other mammals. This is presumably a function of urate's antioxidant properties. Sources of uric acid * In many instances, people have elevated uric acid levels for hereditary reasons. Diet may also be a factor. * Purines are found in high amounts in animal internal organ food products, such as liver.Gout Causes: List of Diet/Food Sources High or Low in Purine Content A moderate amount of purine is also contained in beef, pork, poultry, fish and seafood, asparagus, cauliflower, spinach, mushrooms, green peas, lentils, dried peas, beans, oatmeal, wheat bran and wheat germ.Gout Diet / Low Purine Diet - Limit High Purine foods * Examples of high purine sources include: sweetbreads, anchovies, sardines, liver, beef kidneys, brains, meat extracts (e.g Oxo, Bovril), herring, mackerel, scallops, game meats, and gravy. * Moderate intake of purine-containing food is not associated with an increased risk of gout. * Serum uric acid can be elevated due to high fructose intake , reduced excretion by the kidneys, and or high intake of dietary purine. * Added fructose can be found in processed foods and soda beverages as sucrose, or in some countries, as high fructose corn syrup. Causes of low uric acid Low uric acid (hypouricemia) can have numerous causes. Sevelamer, a drug indicated for prevention of hyperphosphataemia in patients with chronic renal failure, can significantly reduce serum uric acid. Other uric acid facts The crystalline form of uric acid is used as a reflector in certain species of fireflies. The uric acid in urine can also dry in a baby's diaper to form a pinkish powder that is harmless. ----- See also * Hyperuricemia References & Bibliography Further reading Key texts Books Papers *Ahern, F. M., Johnson, R. C., & Ashton, G. C. (1980). Family resemblances in serum uric acid level: Behavior Genetics Vol 10(3) May 1980, 303-307. *Alonso, A., & Hernan, M. A. (2008). "Gout and risk of Parkinson disease: A prospective study": Reply: Neurology Vol 71(1) Jul 2008, 70. *Astakhova, L. N., & Asanova, N. K. (1987). Borderline neuropsychiatric disorders secondary to disorders in purine metabolism in children: Soviet Neurology & Psychiatry Vol 20(1) Spr 1987, 62-71. *Barrera, C. M., Hunter, R. E., & Dunlap, W. P. (1989). Hyperuricemia and locomotor activity in developing rats: Pharmacology, Biochemistry and Behavior Vol 33(2) Jun 1989, 367-369. *Barrera, C. M., Ruiz, Z. R., & Dunlap, W. P. (1988). Uric acid: A participating factor in the symptoms of hyperactivity: Biological Psychiatry Vol 24(3) Jul 1988, 344-347. *Bavaresco, C. S., Zugno, A. I., Tagliari, B., Wannmacher, C. M. D., Wajner, M., & Wyse, A. T. S. (2004). Inhibition of Na-super(+), K-super(+)-ATPase activity in rat striatum by the metabolites accumulated in Lesch-Nyhan disease: International Journal of Developmental Neuroscience Vol 22(1) Feb 2004, 11-17. *Benel, R. A. (1976). Note on the relationship between n Ach and uric acid: Psychological Reports Vol 39(2) Oct 1976, 524-526. *Biersner, R. J., McHugh, W. B., & Rahe, R. H. (1981). Biochemical variability in a team sports situation: Journal of Human Stress Vol 7(3) Sep 1981, 12-17. *Biersner, R. J., McHugh, W. B., & Rahe, R. H. (1984). Biochemical and mood responses predictive of stressful diving performance: Journal of Human Stress Vol 10(1) Spr 1984, 43-49. *Bloch, M., Gur, E., & Shalev, A. Y. (1992). Hypouricemic effect of zuclopenthixol: A potential marker of drug compliance? : Psychopharmacology Vol 109(3) Nov 1992, 377-378. *Bloch, S., & Brackenridge, C. J. (1972). Psychological, performance and biochemical factors in medical students under examination stress: Journal of Psychosomatic Research Vol 16(1) Feb 1972, 25-33. *Bowers, M. B., & Study, R. E. (1979). Cerebrospinal fluid cyclic AMP and acid monoamine metabolites following probenecid: Studies in psychiatric patients: Psychopharmacology Vol 62(1) 1979, 17-22. *Brooks, S. C., Linn, J. J., & Disney, N. (1978). 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The relationship of serum uric acid to intelligence, achievement, and need for achievement in U.S. adolescents, twelve through seventeen years of age, 1966-1970: Dissertation Abstracts International. *Kennett, K. F. (1973). Serum uric acid, intellect, and personality: Dissertation Abstracts International. *Manning, M. R. (1980). Effects of role conflict on registered nurses' physiological, affective, and performance responses: A labortory simulation: Dissertation Abstracts International. *Sayetta, R. B. (1986). The relationship of anxiety and serum uric acid: A model of psychosomatic disease: Dissertation Abstracts International. *Stevens, H. A. (1974). Serum uric acid: A biochemical factor in learning: Dissertation Abstracts International. External links * MedlinePlus - Uric Acid Test * International Kidney Stone Institute * Purine content in food * Computational Chemistry Wiki References Category:Uric acid Category:Nitrogen metabolism Category:Acids Category: